The present invention relates to a printer for forming an image on a continuous-form recording sheet using an electrophotographic method.
Image forming apparatuses using a so-called electrophotographic method such electronic copying machines, have been known in which an electrostatic latent image is formed by exposing a photoreceptor on the surface of a charged photoconductive drum toner is adhered to the latent image for development, transferred to a recording sheet arranged to be synchronously fed with the photoconductive drum and fixed by a fixing unit.
One of these known machines includes a laser beam printer arranged so as to obtain a hard copy of image information by scanning and exposing a charged photoconductive drum by laser beams modulated, in accordance with the image to be developed including figures, characters and the like, using the copy process of the above mentioned electrophotographic method.
The laser beam printer is very useful, because it can be widely used in such a manner that it draws figures of information received by an image reading unit, such as an image scanner. Further, it may be used as an output terminal of a facsimile, and it outputs information at a high speed.
In general, such a laser beam printer is arranged based upon a conventional well-known electronic copying machine, and employs so-called cut-type sheets cut into a predetermined size as a recording medium on which the desired image is formed. A so-called heat roller type fixing unit comprises a pair of fixing rollers composed of a heat roller arranged to be heated with high temperatures and a backup roller arranged to be brought into contact with the heat roller at a predetermined pressure force. The recording medium, upon which an unfixed toner image corresponding to the image information is formed, is caused to pass between the rollers so that it is heated and pressed, whereby the toner is melted and adhered to the recording medium. The desired image is thus formed and fixed on the surface of the recording medium.
In the electrophotographic method, the rotation of a photoconductive drum causes an exposed portion thereof to reach a transfer unit. The toner image is transferred onto the recording sheet, which is fed at predetermined speed identical with a peripheral speed of the photoconductive drum, at the transfer unit. Thus, in this process it is impossible to form images intermittently by interrupting the process.
Therefore, the laser beam printer is provided with a memory capable of storing image information data for at least one page, and when the image information data for one page is completely input therein, the printer outputs them every one page data.
Of course this laser beam printer can be used as an output terminal of a computer. In this case, a continuous-form sheet similar to that used in a conventional line printer is used. The continuous-form sheet, hereinafter referred to as a "continuous sheet", used in the conventional line printer is a so-called fan-folded sheet having a plurality of sprocket holes provided at both side edges at predetermined intervals along a longitudinal direction thereof. The continuous sheet is arranged to be folded along perforated tear lines to enable the sheet to be simply cut off.
When a continuous sheet is employed in a laser beam printer using a heat roll fixing system, the length of a recording sheet feed path from a transferring position of a transferring unit to a fixing position of a fixing unit must be substantially the same as the distance between the perforated tear lines of the continuous sheet. This prevents a disadvantage caused when the laser beam printer stops to feed the continuous sheet after the image forming operations are performed. A page being subjected to a fixing operation is stopped between the pair of fixing, and the unfixed toner, in the process of the fixing operation, remains caught between the pair of the fixing rollers.
More specifically, since the continuous sheet is finally cut off along the perforated tear lines for use, no image must be formed within a predetermined region in the vicinity of the perforated tear lines. Thus in a laser beam printer by which images are formed for each page, the vicinity of the tear lines where no image is formed is arranged to be stopped at the transferring position of the transferring unit. Consequently, when the length of the recording sheet feed path from the transferring position to the fixing position is set to be substantially same as the distance between the perforated tear lines of the continuous sheet, the above disadvantage can be avoided. This is because the vicinity of the tear line where no image is formed is caused to be located at the fixing position of the fixing unit where fixing action is effected wherever the laser beam printer stops to feed the continuous sheet.
In the above structured laser beam printer, a leading edge of the continuous sheet is arranged to be forwardly projected through the pair of fixing rollers by a predetermined amount when the continuous sheet is mounted on the laser beam printer. Therefore, the projected leading edge is heated by the heat roller and curled by the temperature generated by the heat roller, and then the curled sheet is wound around the heat roller. For avoiding this problem, it has been necessary to provide a pressing member for pressing the leading edge projected from the pair of fixing rollers. As a result, the position of the continuous sheet opposed to the transferring position in the transfer unit is located behind the position at which the image forming operation is to be started on the second page of the continuous sheet. In other words, the image forming operation is started from the following page, i.e., the third page. Therefore, the preceding two pages are inevitably wasted.